1. Field
This disclosure is generally related to precoding in Coordinated Multi-Point (CoMP) joint transmission (JT) systems. More specifically, this disclosure is related to a method and an apparatus for designing a precoder in accordance with local parameters.
2. Related Art
Multiple-input-multiple-output (MIMO) systems have tremendous potential in increasing the average throughput in cellular wireless communication systems. The performance gain in channel capacity, reliability, and spectral efficiency in single user (point-to-point) MIMO (SU-MIMO) systems has spurred the inclusion of SU-MIMO in various cellular and wireless communication standards. In cellular networks where spectrum scarcity/cost is a major concern, the ability to reuse spectrum resources is critically important. To further increase spectral efficiency in cellular networks, the spatial diversity of the users can be exploited to allow multi-user MIMO (MU-MIMO) transmissions. The recently emerged 3GPP (3rd generation partnership project) long-term evolution (LTE) and LTE Advanced standards have integrated various MU-MIMO techniques. In addition, CoMP has been promoted as a way to improve coverage, cell-edge throughput, and system efficiency by 3GPP.
In a system implementing CoMP, multiple cells cooperate together to serve certain user equipments (UEs). For example, when a UE is in the edges of multiple cells, it may receive signals from multiple cell sites and its transmitted signals may be received at the multiple cell sites. Hence, if the signaling transmitted from the multiple cell sites is coordinated, the downlink (DL) performance can be improved significantly. In DL CoMP, a number of approaches have been considered, including dynamic cell (or transmission point) selection, coordinated beamforming (CBF), coordinated beam switch, joint transmission (JT), etc. In CBF, the transmission to a single UE is transmitted from the serving cell as in the case of non-CoMP transmission. However, the beamforming is dynamically coordinated between the cells in order to reduce interference. In JT, the transmission to a single UE is simultaneously transmitted from multiple transmission points, across cell sites.
In conventional approaches, precoding in the CoMP JT systems is performed globally. In other words, all cell sites within the CoMP sites are taken into account in the precoder calculation. A number of challenges remain for such global approaches, including reliance on accurate inter-cell phase synchronization, the requirement of high-order matrix calculation, and reliance on relatively accurate channel state information (CSI), which may not be available in the frequency-division duplexing (FDD) cases. Moreover, global precoding approaches also require transmitted data streams and CSI to be shared among all cells in the CoMP set. As the CoMP set gets larger, the need for pairing users also increases, requiring more demodulation reference signal (DMRS) overhead and signaling.